Procedures utilized for the subterranean mining of coal have been greatly improved over the past several decades, both from the standpoint of operational safety on the part of miners as well as from the standpoint of their productivity. However, mining practices still are considered to be labor intensive, a factor significant in the pricing of coal. Additionally, current mining procedures necessarily continue to pose severe occupational safety difficulties. While current techniques of subterranean mining specific to a given strata being worked may represent a variety of technical approaches, the sequence of a given coal mining operation tends to follow a general pattern wherein machines of one variety or another work at the face of a seam to extract coal which then is conveyed outwardly from the mine. During this extraction procedure, there is created a progressively expanding subterranean cavern or chamber. As this procedure is carried out, the structural integrity of the immediately adjacent portions of the cavern roof or supporting portions is jeopardized. Consequently, the roof must be buttressed.
A variety of techniques have been developed and continue to be developed to achieve roof integrity, however, an important and most prevalent one of such techniques provides for the utilization of what are referred to in the art as "roof bolts". Engineering analysis of the function of roof bolts have been set forth in a variety of publications, an example of such being: "Elements of Mining", 3rd ed. by Lewis et al, John Wiley & Sons, Inc. New York. Typically, the procedure for bolting involves first, the carrying out of vertical and predetermined angular drilling through the roof of a recently mined area. This drilling normally will extend at least through a predetermined width of strata. Next, elongate steel bolts are inserted into the bores and anchored therewithin. Next, tension is applied to the bolt to place the rock within which it extends under compression parallel to the bolt. The pattern of bores must be selected such that a proper support for the rock structure is derived. Typically, the bores will be arranged on about a maximum of a four foot mutual centering. The bolts serve a variety of purposes, for instance they may be employed to secure fragments or sections of rock material which are loose and subject to dropping out of place. Their most imperative utilization has been described as "beam building" wherein the bolts are installed in bedded rock to bind the overlying strata together to act as a single beam capable of supporting itself and thus stabilizing the overlying rock. The bolt should be long enough to form a "monolythic" beam which will be self-supporting and not be suspended from the stratum in which the bolts are anchored.
A typical roof bolt is comprised of a length of somewhat flexible steel, the upwardly disposed end of which is operationally coupled to a point anchor present as an expansion shell or slot and wedge type anchor. The steel bolts are inserted within a roof bore and the exposed ends manipulated in connection with a roof engaging plate to expand the anchor and, subsequently, tension the bolt along its length. Over the relatively recent past, resin-anchored roof bolts have found use in roofs composed of strata more difficult to secure i.e. "wet roofs" and the like. These bolts incorporate a thixotropic resin formulation which is activated following insertion within a roof bore to create a tight bond between the bore surface and rod. See "Use of Resin-Anchored Roof Bolts in Adverse Conditions", Mining Congress Journal, Vol. 60, pp. 37-40, January, 1974. Flexibility of all forms of roof bolts is required, inasmuch as mining cavern roof (seam) heights have become quite low, substantially all mining equipment now being fabricated for operation at roof heights as low as about 30 inches.
Installation of the roof bolts in accordance with the most prevailing or current practice is most hazardous. Mining personnel are required to operate under a low unsupported roof portion of the mine and to induce shock producing phenomena in the course of drilling appropriate bores extending through the strata, as well as in the subsequent removal of drill steel and manipulation or stressing of the roof bolts to achieve a necessary beam forming effect. These safety problems as well as associated economic problems have been reported to represent the cause of an estimated 1,114 compensable accidents per year and 1,621 non-compensable accidents per year. The estimated man-days lost per year for this mining function on a non-fatal compensable basis is 60,000 while the non-compensable estimated man-days lost per year has been reported at 2600. In 1973 the five-year total of fatal accidents was set at 50 for roof bolters. See in this regard, "Coming Soon--New Mining Concepts and Equipment for Improved Safety and Production Capabilities", Coal Age, Vol. 78, pp. 137-143, July 1973.
The types of accidents involved in roof bolting vary in scope, certain of them doubtlessly stemming from the more prevalent roof drilling practices. These practices involve the utilization of a hydraulically actuated drilling machine having an arm portion which progressively urges rotating drill steel into the bore at a selected location. Due to the low seam heights now virtually universally encountered, many if not most roofs require bores of a length greater than the height of the cavern in which the operator performs the roof bolting function, the drill steel or drill rods usually being provided as a series of interconnectable components. These components are coupled in chain-like fashion to provide a progressively enlarging length of drill as the bore is formed. See, for example, U.S. Pat. No. 3,554,306; 3,187,825; and 4,009,760. Upon providing a bore of extensive length, it is necessary that the drill rod or steel be removed from the bore. One typical practice for carrying out this removal provides for the attachment of a fork or the like to the lowermost stem portion of the drill and forceably withdrawing the drill steel from the bore. More commonly, the drill steel is removed by hand, the miner grasping the last protruding portion of the drill rod and yanking, hammering and otherwise physically removing it while disconnecting its component sections. Because the bores are not always regular and in view of the extremely rigorous drilling conditions involved in mining, many of the drill rods are partially destroyed as well as bent and become very difficult to remove from the bore by hand. Further complicating the matter, in most instances, the drill steel is extremely hot due to the frictional engagement with the strata through which it has been utilized. A common practice is to connect a chain between the protruding drill component and drill head, following which the head is hydraulically driven downwardly to forcibly remove the drill steel. The occupational hazards involved in drill steel removal, accordingly, become apparant. Further, inasmuch as many sections of drill steel must be abandoned in the bores by virtue of an inability of the miners to remove them, requisite patterns for achieving beam strength within roofs may not be realized, much to the detriment of mining safety, and the lost drill steel must be replaced to add to production expense.
In many instances, certain of the interconnected components of the drill steel are lost by virtue of their frictional engagement within the bore which they have formed. For the most part, the drill steel components are interconnected by slideably mating male and female connections which have no provision for providing tensional coupling to permit forced withdrawal from a bore. Attempts to alleviate this loss steel have generally looked to the use of pins which are driven through mating bores which are formed within the female and male connections. However, such arrangements are found to be impractical in actual mining practice. The miner, generally operating in a posture somewhat near to prone, will remain entirely unappreciative of requirements for carrying punch and hammer first to insert, then to remove the pins as the drill steel is withdrawn from the bore. Such removal within a mine atmosphere is both hazardous and entirely impractical from a human engineering standpoint.
Attempts to alleviate the hazards and difficulties of roof drilling generally have looked to the promise of complete automation of the roof support process. With regard to future promise for such automation, reference is made to the following publications:
"Technological Innovations Abound In Coal Mountains of Appalachia" Coal Age, Vol. 80, pp. 242-250, mid-May 1975. PA1 "Automated Continuous Roof Support", Coal Age Vol. 80, pp. 115-117, July, 1975.
As is apparent from the above discourse, as a prelude to the highly indefinite development of an automated roof supportive system, the industry will recognize considerable advantage in improved efficiencies in roof bolting techniques. In this regard, techniques wherein the time period spent by the miner under unsupported roof during bolting is lessened and the drilling procedure simplified will improve both the safety aspects and efficiency of that required undertaking.